This invention relates to a control rod for use in a fast reactor using a liquid sodium (Na) coolant. More particularly, the present invention relates to a diving bell type control rod equipped with a sodium inflow port for charging sodium into a control element.
Control rods of reactors generally have the construction wherein a plurality of control elements 51 are bundled as shown in FIG. 2.
The control element of the control rod for a fast reactor is constituted by loading sintered pellets of boron carbide (B.sub.4 C) as a neutron absorber into a cladding tube of stainless steel. B.sub.4 C generates helium (He) by (n, .alpha.) reaction with neutrons. The pellet is likely to invite the mechanical interaction with the cladding tube (Absorber-Clad Mechanical Interaction: ACMI) due to swelling. How to cope with both of the increase of the internal pressure in the control element resulting from this helium gas generation and ACMI is very important.
In general, the constructions of the control rods are classified into a seal type which contains the generated helium in a helium plenum disposed inside the control element and a vent type which lets the helium escape from the control element. Further, in the vent type, the B.sub.4 C pellets can be either helium or sodium bonded, and the vent type is therefore classified into a helium bond type and a sodium bond type.
The vent type is more desirable than the seal type because in the vent type the internal pressure in the control element does not increase.
A diving bell type control rod of the helium bond type shown in FIG. 3 is known as one of the vent types, and an enlargement of the vent system region thereof is shown in FIG. 4. In the control element of the control rod, a thin tube of stainless steel called a vent tube 53 is fitted to an intermediate plug 52 as shown in FIG. 3 so as to vent helium generated from pellets 54 from a vent hole 55 through the vent tube 53. The construction of the control element will be explained in further detail with reference to FIG. 4. The control element includes a pellet chamber 57 disposed inside a cladding tube 56 for loading pellets 54, an intermediate plug 52 disposed above the pellet chamber 57, an upper chamber 58 formed above the intermediate plug 52, a vent tube 53 so disposed as to penetrate through the intermediate plug 52 and to allow the pellet chamber 57 to communicate with the upper chamber 58, and a vent hole 55 so formed as to penetrate through the cladding tube 56 located at the lower portion of the upper chamber 58.
The liquid level B of sodium is determined by the balance between the external pressure applied to sodium that is to enter from outside the control element and the internal pressure of the helium gas inside the control element. The vent tube 53 is designed to have an elongated length so that sodium does not enter the pellet chamber 57 from its upper end opening. This diving bell type control rod of the helium bond type has already exhibited proven performance in fast reactors in Japan, and has attained high reliability.
On the other hand, the sodium bond type is effective against ACMI. This is because the sodium bond type can enlarge the gap between the cladding tube and the pellet due to high thermal conductivity of sodium as will be explained below. The B.sub.4 C pellet reaches a high temperature due to the exothermic reaction thereof. In the case of the helium bond type described above, a thermal conductivity of helium is low. Therefore, if the gap between the pellet and the cladding tube is excessively increased, the heat radiation property of the pellet decreases and the temperature of a structural material in the vicinity of the pellet and the temperature of the pellet center rise excessively and undesirably. Contrarily, in the case of the sodium bond type, the thermal conductivity of the gap filled with sodium can be drastically improved and the heat radiation property of the pellet can be improved, as well. Accordingly, a large gap between the cladding tube and the pellet can be secured. In other words, a large initial gap can be secured in the sodium bond type and contact of the pellet with the cladding tube due to swelling of the pellet can be avoided for a long time, so that a longer service life of the control rod can be achieved.
A vertical porous plug type shown in FIG. 5 is a typical example of the conventional sodium bond type control rod. Porous plugs 73, 73 are disposed on upper and lower end plugs 71 and 72, respectively, and sodium is introduced from a lower sodium inlet 74 of the control element and is allowed to flow out from an upper sodium outlet 75. In the control rod of this type, the helium gas is escaped from the sodium outlet 75.
Because the control rod shown in FIGS. 3 and 4 is of the helium bond type, however, the initial gap cannot be secured sufficiently between the pellet and the cladding tube, and there remains the problem that ACMI occurs at a lower burnup of nuclear fuels than in the sodium bond type.
On the other hand, the vertical porous plug type control rod of the sodium bond type shown in FIG. 5 is not free from the possible problem that B.sub.4 C powder produced with cracks of the pellet flows from the lower sodium inlet 74 of the control element into sodium coolant in the primary cooling system.